CNN's What is black carbon? The latest way humans are causing changes in Antarctica doesn't actually say anything about what black carbon is. Neither does The Guardian's Black carbon pollution from tourism and research increasing Antarctic snowmelt, study says.
I assume these are particulates due to incomplete burning of hydrocarbon fuels, and those that reach Antarctica must have a fairly small size in order to get there.
The ones that absorb light most effectively and melt snow may also have some constrains on composition, though maybe "mostly carbon" is all that can be said.
Has there been an attempt to identify the size and composition of black carbon particles that are the primary contributors to snowmelt in Antarctica?